1. Field of the Invention
This invention relates generally to a process for making acrylic coating resins and more specifically to superior solvents useful in the synthesis of high solids acrylic coating resins.
2. Description of the Prior Art
A large variety of acrylic coating compositions are known. Low solids coatings, i.e., those containing about 18 to 40 wt % solids and the balance solvents, have heretofore been developed in which the resins themselves are characterized by high molecular weights, e.g., molecular weights in the range of 20,000 to 40,000. Such high solvent concentrations are required with these high molecular weight resins in order to supply flowability and other properties necessary for ease in applying a uniform coating. Due to strict air pollution regulations, pollution abatement of solvents is of paramount importance. To this end, the industry has expended much effort in an attempt to develop electrostatically sprayable coatings containing high solids contents; that is, coatings having a lower amount of solvents in order to satisfy pollution regulations. Attempts to achieve high solids coatings by merely using more of the conventional high molecular weight resins in the coatings have not been successful since the increased solids content using these resins results in an unacceptably high viscosity, and often the larger amounts of the resins cannot themselves be dissolved. Efforts at developing a "super solvent" for these conventional high molecular weight resins have also not proved to be successful. One prior art approach has been to formulate coatings containing low molecular weight resins (e.g., of about 1,000 to 7,000 weight average molecular weight) in high solids coatings in order to reduce the amount of solvents necessary in the blending for coating applications and, hence, the pollution difficulties associated with the solvents themselves. After application of these coatings to a surface, these coatings are cured to form a polymeric network of higher molecular weight and enhanced physical properties. These high solids acrylic coatings are useful as exterior finish for automobiles, trucks, metal furniture, and as an appliance finish.
K. K. Mitra, "Electrostatic Application of Paint", Paint India, vol. 29, no. 9, pp. 52-56 (1979) indicates that while non-polar solvents (aliphatic and aromatic hydrocarbons chlorinated solvents, terpenes, etc.) can be employed in electrostatically sprayable paints to increase bulk and resistivity, polar solvents are used to control resistivity. The polar group is said to include ketones, alcohols, esters, ethers, ether alcohols, and nitro paraffins, etc. The non-polar group is said to include aliphatic and aromatic hydrocarbons, chlorinated solvents, terpenes, etc.
Also illustrative of prior art, high solids acrylic resins are those disclosed in U.S. Pat. No. 4,276,212 and in European Patent Applications 27,719; 29,594 and 29,683.
Solvents which are indicated to be typical in these references (e.g., those mentioned in European Patent Application 29,594) are: Toluene, xylene, butyl acetate, acetone, methyl isobutyl ketone, methyl amyl ketone, methyl ethyl ketone, butyl alcohol and other aliphatic, cycloaliphatic and aromatic hydrocarbons, esters, ethers, ketones, and alcohols.
In a brochure entitled "Hexyl Acetate for the Coatings Industry" (Enjay Chemical Company), published prior to 1980, use of hexyl acetate as coating solvent in certain specific low solids acrylic coating compositions; in urethane coatings in nitrocellulose coatings; and in baking enamels was disclosed.
U.S. Pat. Nos. 4,075,242 and 4,276,432 disclose the preparation of acrylic-based resins by use of polymerization media containing certain high boiling solvents and disclose the use of ethylene and propylene as co-monomers.
European Patent 29,339 discloses the formation of bifunctional copolymers in which the monomers comprise from 5 to 25 wt % of monoethylenically unsaturated monomers having a glycidyl functionality, from 5 to 25% of monoethylenically unsaturated monomers having hydroxy functionality and 90 to 70 wt % of other monoethylenically unsaturated monomers, with acrylates as well as mixtures of acrylates and vinyl hydrocarbons being preferred. Only monovinyl aromatic hydrocarbons are particularly indicated as useful (e.g., styrene, alpha-methyl styrene, vinyl toluene, t-butyl styrene and chlorostyrene).
U.S. Pat. No. 4,369,296 relates to the production of methyl methacrylate homopolymers (or copolymers with certain copolymerizable vinyl monomers; e.g., styrene and alkyl-substituted styrene) in the presence of from 0.01 to 10 wt % of enol ethers derived from aliphatic or cycloaliphatic aldehydes and ketones.
U.S. Pat. No. 3,271,375 relates to the use, in combination with a free radical polymerizable material, of certain unsaturated heterocyclic organic compounds as molecular weight regulators. The prior art has sought to control the degree of polymerization via chain transfer content (in the preparation of acrylic oligomers for high solids coating resins) by use of relatively inactive solvents such as alkyl aromatics, high boiling ethers and benzyl alcohol. D. Rhum, et al., J. Coatings Tech'n. Vol. 55, no. 703, 75-79 (August 1983).
U.S. Pat. No. 4,532,294 relates to the preparation of acrylic copolymer resins employing polymerization solvents comprising certain alkanoic acid alkyl esters together with hydroxy-substituted alkyl (meth)acrylate and non-hydroxy substituted alkyl (meth)acrylate monomers, and in optional presence of additional monomers comprising monovinyl aromatic hydrocarbons. Among the non-hydroxy substituted alkyl (meth)acrylate monomers which may be employed are (meth)acrylates as well as mixtures of acrylates and vinyl hydrocarbons.
U.S. Pat. No. 3,926,925 relates to novel interpolymers containing an olefin (an alpha-olefin, a 2-alkyl-1-olefin and a vinyl aromatic), polar monomers, such as an alkyl acrylate, and a fumarate ester or a maleic acid derivative which are prepared with a catalyst system of an alkyl aluminum halide and an organic peroxide. U.S. Pat. No. 3,959,225 relates to a thermally-staged process for preparing alternating interpolymers of one or more polar monomers and one or more mono- or polyolefins in which a polar monomer-Lewis Acid complex is reacted with an olefin in the presence of an active oxygen compound. The olefins discussed as useful in U.S. Pat. No. 3,959,225 are certain Type I olefins and Type III olefins.
U.S. Pat. No. 3,968,148 relates to oligomers of 1-alkenes and derivatives of acrylic acids.